Letters - July 2007

Philately and chemistry

Over the years there has been a number of postage stamps issued to celebrate scientific discoveries or honour well-known scientists and these can be used as simple yet powerful teaching tools in the classroom.1 I use a chemistry-related postage stamp from Spain (pictured right) with my first-year undergraduate students to introduce several concepts related to the Periodic Table. I set this as a collaborative learning activity (with groups of three students) and discuss with the students their answers. This example could also be used as a problem for students to tackle individually or to start a discussion during a lecture.

I begin by telling my students about the postage stamp, which was issued in Spain on 2 February 2007 - the Spanish Year of Science. Inspired by the stamp, I set them three challenges.

Comment on why this stamp has been issued (anniversary, symbolism of figure etc).

Explain what is represented by the spaces coloured in blue, red, yellow and green, and comment on their dimensions.

Comment on the importance of the 'little white squares', represented in the red and yellow zones, in the history of the development of the Periodic Table.

By working individually, in groups or by class discussion the students arrive at the solutions.

The stamp was launched by Correos (General Post Office in Spain) in 2007 in honour of the 100th anniversary of the death of Dmitri Mendeleev. The stamp refers to the periodic classification of the elements proposed by Mendeleev in 1869. As he attempted to classify the elements according to their chemical properties, he noticed patterns that led him to postulate his Periodic Table which described elements according to both weight and valence and which, if arranged according to their atomic weights, exhibited an apparent periodicity of properties.

The Periodic Table has evolved so that now the periodicity is explained with respect to the variation of electronic configurations with the atomic number. In accordance with the ground-state electron configurations, the blue area on the stamp represents elements filling s-orbitals, the red rectangle represents d-transition elements, the yellow area represent elements filling p-orbitals and the green rectangle represents f-transition elements. Assuming that the Periodic Table is depicted in the stamp, and by taking each little white square to represent an element, the stamp's 'Table' is seven periods in height (the f-block elements qualifying as elements of the sixth and seventh periods) with two columns of elements in the blue zone, 10 in the red zone, six in the yellow zone, and 14 in the green zone, which tallies with any modern, standard form of the Periodic Table.2

Unlike other contributors to the development of the Periodic Table, Mendeleev predicted the properties of elements yet to be discovered and made an accurate prediction of the properties of scandium, technetium, gallium and germanium (the little white squares on the stamp, from left to right) which came to fill the spaces in his Table.3

This example shows how a simple postage stamp can serve as inspiration for questions on topics in chemistry. Discussions of this kind may encourage students to explore topics in more depth. (Note: this stamp was designed by the Spanish chemist Javier García Martínez. He was inspired by the work of Piet Mondrian, a Dutch neoplasticist painter (1872-1944). )

Acknowledgement: I am thankful for the support provided by the Universidad Politécnica de Madrid under the project, New methodologies for improving the teaching-learning process in chemistry.

Blood - be aware

Readers of Colin Baker's excellent Exhibition chemistry article on how to prepare his 'blood mousse' demonstration should be aware that appropriate precautions need to be taken when handling mammalian blood.4 The use of blood is not banned in schools or colleges but there are hygiene and disposal considerations which need to be addressed and followed. Subscribers to CLEAPSS can find this information in section 14.4 of the CLEAPSS laboratory handbook.

In addition, I have done similar hydrogen peroxide decompositions (with other catalysts) and by having starch/iodide paper in various positions above the foam, I have found that hydrogen peroxide aerosol is projected into the atmosphere during the reaction. This is why CLEAPSS advises that the room is well-ventilated.

In Baker's demonstration the aerosol will be contaminated with mammalian blood. Open vessels, in this case, should not be used, so the reaction should be done in a fume cupboard or inside a large, transparent plastic bag.

Although it would not make such a brightly-coloured demonstration, liver is a more acceptable source of catalase.